For almost a century, astronomy students have been learning that little mnemonic to help them remember how the stars are classified: with the letters O-B-A-F-G-K-M. The system was devised by Annie Jump Cannon, who was born 150 years ago today.

Until the development of spectroscopy, about the only way to classify stars was by their brightness or their position in the sky.

But spectroscopy changed that. By splitting a star’s light into its individual wavelengths or colors, it revealed the star’s composition, its temperature, its motion, and much more. Since they now knew a lot more about the stars, astronomers needed a way to classify them that would help them better understand how stars work.

Harvard College Observatory hired Cannon and other women to pore over hundreds of thousands of spectra. Cannon was particularly good at it. During her 40-year career, she catalogued more than 300,000 of them.

Before Cannon, a couple of people had developed ways to categorize the stars based on their spectra. One used the letters of the alphabet up through “N.” But Cannon saw a better way. She kept the letters from the earlier system, but rearranged them based on a star’s color, which corresponds to its temperature. In her system, blue-white stars — the hottest — come first, followed by white, yellow, orange, and red.

Cannon’s system remains in use today — giving students an easy way to remember how to classify the stars.

Script by Damond Benningfield, Copyright 2013

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